A third rail is a method of providing electricity to power a railway by means of a continuous rigid conductor mounted alongside the railway track or between the rails. It is used typically in a mass transit or rapid transit system, which has alignments in own corridors, fully or almost fully segregated from the outside environment. A list of lines or networks equipped with a third rail is provided further below. Third rail systems generally supply direct current to power the trains. In the early 1900's, the third rail electric system was used to power early rollercoasters such as the Rough Riders (rollercoaster) in Coney Island, New York.

The third rail system of electrification is unrelated to the third rail used in dual-gauge railways.

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Third-rail electric systems are, apart from on-board batteries, the oldest means of supplying electric power to trains on railways using own corridors, particularly in cities. Overhead power supply was initially almost exclusively used on tramway-like railways, though it also appeared slowly on mainline systems. (This statement describes the general trend; early particular cases may have been different.)

An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Industrial Exhibition of 1879. This pioneer electric railway had its third rail placed between running rails. At some early electric railways, though, one of the running rails could be the current conductor, as was the case of the 1883-opened Volk's Electric Railway in Brighton. Soon it was given an additional power rail in 1886 (the railway is still operating). The Giant's Causeway Tramway followed, equipped with an elevated outside third rail in 1883 (but later converted to overhead wire pickup). The first railway to use the central third rail was the Bessbrook & Newry Tramway, opened in Ireland in 1885 but now, like the Giant's Causeway line, closed. Also in the 1880s third-rail systems began to be used in public urban transport. Trams were first to benefit from it, but they used conductors built in conduit below the road surface (cf. Conduit current collection), and usually on selected parts of the networks. This was first tried in Cleveland (1884) and in Denver (1885) and later spread to many big tram networks (e.g. Manhattan, Chicago, Washington DC, London, Paris — all closed) and Berlin (The third rail System in the city was abandoned in the first years of the twenties century — after heavy snowfall)) .

A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890 (now part of the Northern Line of the London Underground). In 1893 the world's second third-rail powered city railway opened in Britain — the Liverpool Overhead Railway (closed 1956 and dismantled). The first US third-rail powered city railway in revenue use was the 1895-opened Metropolitan West Side Elevated, which soon became part of the Chicago 'L'. In 1901, Granville Woods, a prominent African-Americaninventor, was granted a Template:US patent, covering various proposed improvements to third rail systems. This has been cited to claim that he invented the third rail system of current distribution. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's Template:US patent of 1882, and third rails had been in successful use for over a decade, in installations including the rest of Chicago 'elevateds', as well as these in Brooklyn, New York (if not to mention the development outside the US). To what extent Woods' ideas were adopted is thus a matter of controversy.[2]

In Paris, in 1900, third rail appeared in the mainline tunnel connecting the Gare d'Orsay to the rest of the CF Paris-Orléans network. Mainline third rail electrification was later expanded to some suburban services in the French capital.

Top contact third rail (cf. below) seems to be the oldest form of power collection. Railways pioneering in using other, less hazardous types of third rail, were the New York Central Railroad on the approach to its NYC's Grand Central Terminal (1907 — another case of a third-rail mainline electrification) and the Hochbahn in Hamburg (1912) — both had bottom contact rail. However, the Manchester-Bury Line of the Lancashire & Yorkshire Railway tried the side contact rail (1917). These technologies appeared in wider use only at the turn of the 1920s and in the 1930s at, e.g., large-profile lines of the Berlin U-Bahn, the Berlin S-Bahn and the Moscow Metro. The Hamburg S-Bahn is using a side contact rail with 1200 V dc since 1939.

In 1956 world's first rubber-tired railway line was opened. This was Line 11 of Paris Metro. Power rail evolved into a pair of guiding rails required to keep the bogie in proper position on the new type of track. This solution was modified on the 1971-opened Namboku Line of Sapporo Subway, where a centrally placed guiding/return rail was used plus one power rail placed laterally as usually on steel rail railways (cf. photo).

The third rail technology at street tram lines has recently been revived in the new system of Bordeaux (2004). This is a completely new technology (cf. below).

Third rail, being the older of the two electric current supply methods, is by no means obsolete. There are, however, countries (particularly Japan, South Korea, India, Spain) more eager to adopt overhead wiring to their urban railways. But in the same time there were (and still are) many new third rail systems built elsewhere, including technologically advanced countries (i.e. Copenhagen Metro, Taipei Metro, Wuhan Metro). Bottom powered railways (it may be too specific to use the term 'third rail') are also usually these having rubber-tyred trains, no matter if it is a heavy metro (except two other lines of Sapporo Subway) or a small capacity people mover (PM). Practically the only type of railways where third rail is no longer used in new systems is regional and long distance rail, which require higher speeds and voltages.

The first idea for feeding electricity to a train from an external source was by using both rails on which a train runs, whereby each rail is a conductor for each pole insulated by the sleepers.
This method is used by most model trains, however it does not work so well for large trains as the sleepers are not good insulators, furthermore the use of insulated wheels or insulated axles is required. As most insulation materials have worse static properties compared with metals used for this purpose, this results in a less stable train vehicle.
Nevertheless, it was sometimes used at the beginning of the development of electric trains. The following systems used it:

The third rail is usually located outside the two running rails, but occasionally runs between them. The electricity is transmitted to the train by means of a sliding "shoe" (pick-up or contact shoe) which is held in contact with the rail. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (called side running) or bottom (called bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as "top running". When the shoe slides on the bottom it is not affected by the build-up of snow or leaves.

As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tires, as on parts of the Paris Métro, Mexico City Metro and Santiago Metro, as well as on all of the Montreal Métro, live guide bars must be provided to feed the current. The return is effected through the rails of the conventional track between these guide bars (see rubber-tired metro). Another design, with a third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails), is used by a few steel-wheel systems, see fourth rail. The London Underground is the largest of these, see railway electrification in Great Britain.

In line M1 of the Milan underground, the third rail is used as the return electrical line (with potential near the ground) and the live electrical connection is made with a sliding block on the side of the car contacting an electrical bar located next to the railway (between the railway and the opposite direction railway) approximately 1 m (3') above the rail level. In this manner there are four rails. In the northern part of the line the more common overhead lines system is used.

The third rail is an alternative to electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V (Hamburg S-Bahn), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (cf. below, "Compromise systems").

One method for reducing current losses (and thus increase the spacing of feeder/sub stations — a major cost in third rail electrification) is to construct the conductor rail of a hybrid aluminium/steel design (or composite conductor rail). The aluminium, which is a better conductor of electricity, combined with a running face of stainless steel, which gives better wear, aims to match the existing steel conductor rails.

There are currently several marketed ways of attaching the stainless steel to the aluminium. The oldest is a co-extruded method, where the stainless steel is extruded with the aluminium. This method has suffered, in isolated cases, from de-lamination (where the stainless steel separates from the aluminium); this is said to have been eliminated in the latest co-extruded rails. A second method is an aluminium core, upon which two stainless steel sections are fitted as a cap and linear welded along the centre line of the rail. A third method involves a one-piece cap over the aluminum core with side welds affixing stainless steel strips captured in the aluminum extrusion. Both welded rail configurations provide a wrap-around stainless steel cap, preventing collector shoe contact with the soft aluminum. Because aluminium has a higher coefficient of thermal expansion than steel, the aluminium and steel must be positively locked to provide a good current collection interface. A third method rivets aluminum bus strips to the web of the steel rail. The photo on the right depicts such a rail.

Third-rail systems are cheaper to install than overhead wire systems, less prone to weather damage (other than flooding and icing, which cause major problems), and better able to fit into areas of reduced vertical clearance, such as tunnels and bridges. In many countries they were perceived as key means of reducing construction costs of tunnels, hence their popularity at underground railways.

Third-rail systems cause less visual intrusion: they do not need overhead lines, which some people perceive as unsightly. Singapore, for example, has banned overhead wires on lines outside tunnels. Urban street railways have been built, for example in Bordeaux, that carry the conductor rail within a slotted box in the center of the track (conduit current collection), primarily to avoid unsightly overhead wires and poles. These resemble the cable slot for a street cable car as seen in San Francisco. Rather than a mechanical grip, an insulated electrical pickup extends into the slot.

Third-rail systems are more robust than overhead line systems, as the conductor rail is able to take higher mechanical forces than the contact wire of an overhead line system. The shoegear on a train is designed to shear off if it hits the conductor rail too hard, but as a train has many sets of shoegear, it is able to continue its journey. By contrast a pantograph is more likely to get tangled up in the overhead wires and not be able to continue its journey.

Many railways use third rail and DC power, even where overhead lines would otherwise be practical, due to the high cost of retrofitting. Every expansion of such system must cope with the problem of compatibility. It usually leads for the choice of already existing technology.

An unguarded electrified rail carrying more than 50 volts is a safety hazard, and some people have been killed by touching the rail or by stepping on it while attempting to cross the tracks. However, such incidents are usually the result of carelessness on the part of the victim. The principal hazard is probably associated with level crossings. While their number on third rail lines is normally reduced to none, they still occur at some systems, particularly on rural and suburban portions of the network. One notable example of a Metro line running a third rail at ground level is the outer ends of the present Brown Line and Pink Line of the Chicago 'L', running on street level in a densely populated neighborhood. The conductor is discontinued in the level crossing area. Pedestrians may be discouraged from trespassing into railway area by means of perforated panels difficult to step on (cattle-cum-trespass guards). They are laid between rails alongside the road.

Intercity ground level third rail systems are the norm in the southeast of England, level crossings are handled in a fashion similar to the Chicago system. A few interurban electric railways attempted to utilize third rail in the USA, these were quickly abandoned as impractical outside of New York City commuter lines such as the Long Island Rail Road. Both the US and UK intercity systems address safety through extensive fencing and warning signage.

There are urban legends that people have died while urinating on the third rail (the urine stream supposedly completes an electrical circuit that electrocutes the victim); a non-continuous stream has been demonstrated by MythBusters to be unable to conduct electricity [3]. This myth may be partially perpetuated by a 1977 incident that occurred in Chicago where an intoxicated pedestrian suffered a fatal electrocution injury while trespassing to urinate on the grade-level CTA Brown Line right-of-way near Kedzie Avenue. However, the death occurred as a result of the passenger making physical contact with the third rail (not as a result of an electrical circuit being completed via his urine stream)[4].

A new tramway system in Bordeaux, France surmounts the safety problem by using a third rail divided into insulated segments only a few metres long. Each segment is live only while completely covered by a tram, so there is no risk of a person or animal coming into contact with a live rail (see Third-rail power for trams and stud contact electrification for more information). This system would not be suitable for higher speeds, and the cost of breaking the live rail into short sections is considerable. This system was developed mainly for aesthetic reasons, to avoid overhead wires in front of the town hall.

Other safety precautions can be made to reduce the risk of the third rail. Many subway systems, such as the BART and the Washington Metro, use sturdy sheaths to cover its third rails and always place the rail on the further side of the track away from where passengers would normally be. If someone falls on the tracks, there is room to return safely to the platform (or crawl under the platform) without the danger of stepping on the third rail.

A relatively low voltage is necessary in a third-rail system — otherwise, electricity would arc from the rail to the ground or the running rails — but the resulting higher current (sometimes upwards of 3,000 amperes) causes more proportional voltage drop per mile, meaning that electrical feeder sub-stations have to be set up at frequent intervals along the line (generally no more than 10 miles (Template:Rnd/+ km) apart), increasing operating costs. A 1992 report, prepared for the California Department of Transportation by Morrison-Knudsen Corporation, states that typical spacing of substations supplying a 600 volt DC track is one mile apart, and the cost of electrifying tracks with third rail is nearly double that of overhead catenary.[1] The low voltage also means that the system is prone to overload, which makes such systems unsuitable for freight or high-speed trains demanding high amounts of power. These limitations of third-rail systems have largely restricted their use to mass transit systems. Capacity is also limited by speed restrictions. Testing on the southern region of British Rail during the late 1980's/early 1990's established that third rail could handle 160 km/h (100 mph)and that third rail could provide reliable current collection. Testing at greater speeds was not tried as the track tested has a maximum speed of 160 km/h (100 mph).[2]

By comparison, overhead wires can provide 25kV or even 50kV, and can take roughly ten times the power[citation needed].

Junctions and other pointwork make it necessary to leave gaps in the live rail at times, as do level crossings. This is not usually a problem, as most third-rail rolling stock has multiple current collection shoes along the length of the train, but under certain circumstances it is possible for a train to become "gapped" — stalled with none of its shoes in contact with the live rail. When this happens, it is usually necessary for the train to be shunted back onto a live section either by a rescue locomotive or another service train, although in some circumstances it is possible to use jumper cables to temporarily hook the train's current collectors to the nearest section of live rail. Especially given that gapping tends to happen at complex, important junctions, it can be a major source of disruption. On the Chicago Transit Authority system, the jumper cables are known as stingers; they are insulated poles with a wired contact that may be manually pressed against contact shoes to restart a gapped train. Other such problems are implementation-specific, usually have workarounds. Another infrastructure restriction of third rail is that the rail and its safety cover decreases the structure gauge and in turn the loading gauge, potentially blocking access to certain types of equipment.

When David Gunn became General Manager of the Washington DC Metro Rail system, he publicly proposed to alleviate crowding by running much more frequent trains as two-car trains instead of the practice the transit authority had of running four-car trains. He had to publicly drop this idea, with some embarrassment, when it was pointed out to him that two-car trains can only operate in specific areas of the system, because each car only has one "shoe", on the same side of the car, and even with the practice of having each car pointing in the opposite direction so that there is a shoe on each side of the train, there are many places in the system where a two car train would end up with both "shoes" unable to reach a third rail, stranding the train.

The information presented above regarding the DC Metro is inaccurate.

Each Metro car has one pickup shoe mounted on each side of each truck. That is two shoes per truck and four shoes per car. The four shoes on each car are wired together so if one shoe is hot then all four shoes are hot. High voltage is not carried between cars.

Metro cars come in married pairs and it is not possible to turn one car around. A train could lose the shoes on one side but that has nothing to do with the way each car is facing. On a hot summer day the 3rd rail can shift, causing all of the shoes on one side of a passing train to shear off.

The main problem with a deuce (a single married pair) is that there is no redundancy. If one air compressor of a six car (three pair) train fails there are two more to keep the train moving. In a deuce there is only one air compressor. There is also only one train control system, one radio, etc.

There are sections of 3rd rail near sub-stations that have a gap longer than the distance between the shoes of one car. These "non-bridgable gaps" prevent a car from bridging two sections of rail together, thus possibly energizing a piece of rail that should remain de-energized. A non-bridgable gap will de-energize one car of a deuce but the other car will likely start the train moving as long as the air pressure has not fallen.

Crossovers are another matter since they have many short 3rd rail gaps. A deuce that starts a crossover move and is then stopped could easiliy wind up with no shoes on the rail. If the car is on a grade the operator may be able to coast onto live rail but coasting backwards is inherently dangerous. If the operator does not act quickly the air pressure will drop and the emergency brakes will come on. You will then have a train stuck in the crossover, possibly preventing a rescue train from reaching it.

Most transit systems will not run a single married set due to the lack of redundancy. Philadelphia had a few single section "Almond Joy" cars on its Market Frankford line that it could safely operate in pairs. Since both units were complete there was redundancy even in a two car train. These trains also had a conductor station so they could collect fares on the car at night.[citation needed]

Older systems adopted top-contact third rail before they realised that there would be problems with leaves, etc., while newer systems have learned from this mistake and use side or bottom contact. However, some relatively new systems in North America, such as the TTC in Toronto, use top-covered top-contact third rails on above-ground portions of its subway system; rarely is the system delayed by electrical problems even after heavy snows. Rather, problems generally arise in other aspects of the system (frozen switches for example) long before snow interferes significantly with electrical pickup. Some systems are less susceptible to this problem due to having mostly underground trackage, or less severe weather.

Several systems use third rail for part of the system, and other systems such as overhead catenary or diesel power for the remainder. These may exist because of the connection of separately-owned railways using the different systems, local ordinances, or other historical accidents.

Also in New York City, due to a prohibition on diesel emissions in tunnels, Metro-North, Long Island Rail Road and Amtrak use diesel locomotives that can also be electrically powered by third-rail. This kind of locomotive (for example the P32AC-DM or the EMD/Siemens built DM30AC of LIRR), can transition between the two modes while underway. The third-rail auxiliary system is not as powerful as the diesel engine, so on open-air (non-tunnel) trackage run the engines typically run in diesel mode, even where third rail power is available. This does not hold true for the DM30ACs, where the electric mode is much more powerful than the diesel.

In Manhattan, New York City, and in Washington, D.C., local ordinances required electrified street railways to draw current from a third rail and return the current to a fourth rail, both installed in a continuous vault underneath the street and accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead. Some sections of the former London tram system also used the conduit current collection system, also with some tramcars that could collect power from both overhead and under-road sources.

In Washington, D.C., conductor bars for both positive and negative power were located in the conduit. A plow hanging from the car had large pick-up shoes that contacted the conductor bars on either side. The polarity could be switched and there was no way to know which side was which.

In Washington, D.C., there was a man in the plow pit who changed the plow on the car. He would take two connector plugs from the car electrical system and plug them into the two sockets on the plow or into to two jacks mounted on the car body, one for the trolley pole and one for the body and wheels.

A man on the street would put the poles up or pull them down. Automatic trolley retrievers were used in some locations to raise and lower the poles but a pitman was still needed to remove or attach the plow. The automatic trolley system had a "Trolley" switch labeled "Up" and "Down". Pushing the switch "Down" caused an air mechanism in the center of the retriever to wind the pole onto the roof, where it was caught by a latching assembly, which replaced the conventional pole hook. Pushing the switch "Up" released the latch on the roof, allowing the pole to fly upward into a pan that directed its shoe onto the wire.

The Blue Line of Boston'sMBTA uses third rail electrification from the start of the line downtown to Airport, where it switches to overhead catenary for the remainder of the line to Wonderland. Dual power supply method was also used on some US interurban railways that made use of newer third rail in suburban areas, and existing overhead streetcar (trolley) infrastructure to reach downtown, for example the Skokie Swift in Chicago.

The Eurostar uses overhead electrical power (at 25 kV AC) in the Channel Tunnel and along High Speed 1, with a pantograph height change required between HS1 and the Channel Tunnel (which is a unique height). (The trains are also required to cope with height changes for the French urban and TGV lines). In the south-east at Fawkham Junction, a transition is made on-the-fly to 750 V DC for the remainder of the journey through the London suburbs, on the standard commuter lines into Waterloo using the third rail system. Since 14 November 2007, upon commencement of fare-paying services on the completed High Speed 1, there is overhead electricity all the way into St. Pancras International station.

The older lines in the west of the Oslo T-bane system were built with overhead lines (some since converted to third rail) while the eastern lines were built with third rail. Trains operating on the older lines can operate both with third rail and overhead lines. To mitigate investment costs, the Rotterdam Metro, basically a third-rail powered system, has been given some outlying branches built on surface as light rail (called 'Sneltram' in Dutch), with numerous level crossings protected with barriers and traffic lights. These branches have overhead wires. Similarly, in Amsterdam one 'Sneltram' route goes on Metro tracks and passes to surface alignment in the suburbs, which it shares with standard trams. In most recent developments, the RandstadRail project also requires Rotterdam Metro trains to run under wires on their way along the former mainline railway to The Hague.

The newly built tramway in Bordeaux (France) uses a novel system with a third rail in the center of the track. The third rail is separated into 8 m (26 ' 3 ") long conducting and 3 m (9 ' 10 ") long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the tram (activated by a coded signal sent by the train) and switch it off before it becomes exposed again. This system (called "Alimentation par Sol" (APS), meaning "current supply via ground") is used in various locations around the city but especially in the historic centre: elsewhere the trams use the conventional overhead lines, see also ground-level power supply. In summer 2006 it was announced that two new French tram systems would be using APS over part of their networks. These will be Angers and Reims, with both systems expected to open around 2009–2010.

The French Fréjus line to Modane has been electrified with 1500 V DC third rail, and later converted to overhead wires at the same voltage. Stations had overhead wires from the beginning.

Despite various technical possibilities of operating stock with dual power collecting modes, the desire to achieve full compatibility of entire networks seems to have been the decisive cause of conversions from third rail to overhead supply (or vice versa).

Selected suburban corridors in Paris, focusing at Gare Saint-Lazare, Gare des Invalides (both CF Ouest) and Gare d'Orsay (CF PO), were electrified from 1924, 1901, 1900 respectively. They all changed to overhead wires by stages after they became part of a wide scale electrification project of the SNCF network (the 1960s–70s).

In Manchester area, the aforementioned Bury Line (originally L&YR) was first electrified with overhead wires (1913), then changed to third rail (1917, cf. Railway electrification in Great Britain) and again in 1992 to overhead wires in the course of its adaptation for the Manchester Metrolink. Trams in city centre streets, carrying collector shoes projecting from their bogies, were considered too dangerous for pedestrians and motor traffic to attempt dual-mode technology (in Amsterdam and Rotterdam Sneltram vehicles go out to surface in suburbs, not in busy central areas). The same thing happened to the West Croydon — Wimbledon Line in Greater London (originally electrified by the Southern Railway) when Croydon Tramlink was built (opened 2000).

Three lines of five making up the core of Barcelona Metro network changed to overhead power supply from third rail. This operation was also done by stages and completed in 2003.

Quite the opposite thing took place in London. The South London Line of the LBSCR network (between Victoria and London Bridge Stations) was electrified with catenary in 1909 — the system was later extended to Crystal Palace, Coulsdon North and Sutton. In the course of mainline third rail electrification in south-east England, the lines were converted accordingly by 1929.

In 1976–1981 the third-rail Viennese U-Bahn U4 Line substituted the Donaukanallinie and Wientallinie of the Stadtbahn, built c1900 and first electrified with overhead wires in 1924. This was part of a big project of consolidated U-Bahn network construction. The other electric Stadtbahn line, whose conversion into heavy rail stock was rejected, still operates under wires with light rail cars (as U6), though it has been thoroughly modernised and significantly extended. As the platforms on the Gürtellinie were not suitable for raising without much intervention into historic Otto Wagner's station architecture, the line would anyway remain incompatible with the rest of the U-Bahn network. Therefore an attempt of conversion to third rail would have been pointless. In Vienna, paradoxically, the wires were retained for aesthetic (and economic) reasons.

The already discussed Skokie Swift of Chicago 'L' changed to third rail in 2004, to make it compatible with the rest of the system.

The reasons for building the overhead powered Tyne & Wear Metro network roughly on lines of the long-gone third-rail Tyneside Electrics system in Newcastle area are likely to have roots in economy and psychology rather than in the pursue of compatibility. At the time of the Metro opening (1980) there were no third-rail light rail vehicles on the market and the latter technology was confined to much more costly heavy rail stock. Also the far-going change of image was desired: the memories of the last stage of operation of the Tyneside Electrics were far from being favourable. This was the construction of the system from scratch after eleven years of ineffective diesel service.

In principle, a railway can be electrified with an overhead wire and a third rail at the same time. This was the case, for example, at Hamburg S-Bahn between 1940 and 1955. A modern example is Birkenwerder Railway Station near Berlin, which has third rail on both sides and overhead wire. However, such systems have problems with the influence of the different supplies. If one supply is DC and the other AC, an undesired premagnetization of the AC transformers can occur. For this reason, double electrification is usually avoided.

The border station of Modane, along the French-Italian Fréjus railway was electrified with both 1500 V DC third rail for French trains and with overhead wires (initially three-phase, later 3000 V DC) for Italian trains. When the French part of the line was converted to overhead wires, the voltage of the wires was dropped to 1500 V DC. Now Italian trains run in Modane feed with 1500 V DC instead of 3000, with half of their power.

In 1906, the famous Lionel electric trains became the first model trains to use a third rail to power the locomotive. Lionel track uses a third rail in the center, while the two outer rails are electrically connected together. This solved the problem two-rail model trains have when the track is arranged to loop back on itsef, as ordinarily this causes a short-circuit. (Even if the loop was gapped, the locomotive would create a short and stop as it crossed the gaps.) Lionel electric trains also operate on alternating current. The use of alternating current means that a Lionel locomotive cannot be reversed by changing polarity; instead, a short spike of power reverses a relay within the locomotive while it is stopped.

By contrast, most model train sets today use only two rails, and supply locomotives with direct current. The voltage and polarity of the current controls the speed and direction of the train. A growing exception is DCC (Digital Command Control), where bi-polar DC is delivered to the rails at a constant voltage, along with digital signals that are decoded within the locomotive; the signals carry addresses that indicate which locomotive is being commanded when multiple locomotives are present on the same track.

Some model railroads realistically mimic the third rail configurations of their full-sized counterparts; such models may or may not actually draw power from the third rail (most do not).